Inkjet printers typically have an input media source such as a media stack in an input tray, an output tray, a media path between the input source and the output tray, and an inkjet printing apparatus located along the media path at a print area. The printing apparatus can comprise one or more inkjet printheads with nozzle arrays which emit droplets of ink onto the print media at the print area A media handling apparatus is provided to pick the input media from the input source, feed the picked medium along the media path to the print area, and eject the picked medium onto the output tray after printing operations on the medium are completed.
In a typical sheet-fed printer using print media in sheet form, such as paper, a pick roller is employed to pick the top sheet of print media from the input tray and advance the picked sheet along the media path toward the printing apparatus. This is illustrated in the diagrammatic view of FIG. 1, wherein the pick roller 10 with associated pinch roller 13 has picked the sheet 12 from an input source (not shown), and pulled the sheet around the input guide 15 with curved guide surface ISA. The sheet handling apparatus further typically includes a feed or drive roller 14 and a forward pinch roller 16 which create a nip into which the leading edge of the picked sheet is fed by the pick roller along guide 18. The print zone at which printing operations are conducted is typically located on the media path just downstream of the pinch roller 16. Stresses are applied to the picked sheet at the print zone for media shape control and wet cockle control.
A problem arises in that the trailing edge 12A of the picked sheet is unconstrained after leaving the pick roller. Because of the stresses applied to the picked sheet in the print zone, the unconstrained shape of the sheet after leaving the pick roller is significantly rotated about the forward pinch roller 16. This is illustrated in FIG. 1, in which the constrained state prior to leaving the pick roller 10 and pinch roller 13 is indicated as sheet 12 with trailing edge 12A, and the unconstrained state is indicated as sheet 12′ with trailing edge 12A′. This results in a rapid print medium shape change in stiff media that can cause an effective overfeed as seen by the print head just downstream of the nip between the drive roller and pinch roller. The effective overfeed causes a print defect, known as a “bottom of form” (BOF) print defect. This print defect is often quite visible on images printed on premium photo paper, for example.
Another cause of print defects for media handling apparatus incorporating separate roller wheels instead of solid rollers, is that, as the print medium is compressed under pinch rollers, energy is stored in the medium by deforming the print medium-around the rollers. This is illustrated in the cross-section view of FIG. 2, taken transversely to the media path. Here the pick roller structure and the pinch roller structure is defined by three spaced pick roller wheel/pinch wheel pairs, 10A/13A, 10B/13B and 10C/13C. The deformation of the medium 12 in the regions between the wheel pairs is illustrated in exaggerated form in FIG. 2. This deformation can cause overfeeding, especially on stiff medias, when the trailing edge of the medium leaves the nip between the drive and pinch rollers.
These print defects will generally be described as “trailing edge” print defects, i.e. those print defects occurring when the trailing edge of the print media passes some point, e.g. a pinch point or the pick roller.
It would therefor be an advantage to provide a technique to minimize or eliminate trailing edge print defects in printing systems using media handling apparatus with one or more rollers.